Dual slot wide band antenna



May 7, 1957 N. E. LINDENBLAD DUAL SLOT WIDE BAND ANTENNA Filed Sept. 27. 1950 2 Sheets-Sheet 1 M18 EMEZIEEZJM ATTORNEY May 7, 1957 N. E. LINDENBLAD 2,791,769

DUAL SLOT WIDE BAND ANTENNA 2 Sheets-Sheet 2 File d Sept. 27. 1950 3 143 l 1273 R n 75: A 1619 1 15-165 2 /66 17:? IyVEN-rOR I 1 N118 E. lmdellbla AiDRNt United States Patent Ofi 2,791,769 Patented May 7, 1957 DUAL SLOT WIDE BAND ANTENNA Nils Erik Lindenblad, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 27, 1950, Serial No. 186,969

22 Claims. (Cl. 343-770) The invention relates to antennas and particularly to a slot antenna having an operating band width which is greater than that obtained heretofore in slot antennas.

The slot antenna is well known in the art, typical examples being shown and described in U. S. Patents 2,414,266 to N. E. Lindenblad, 2,425,303 to P. S. Carter and 2,487,622 to R. S. Wehner. Such antennas have been adapted for certain uses, perhaps the most important class of which are those intended for installation on highspeed aircraft. There remain, however, a considerable number of applications in which the construction of a slot antenna obviously would be quite advantageous but certain electrical or mechanical limitations preclude the use thereof. The most serious limitation encountered in slot antennas is the relatively limited frequency band width obtainable with prior art arrangements. Numerous suggestions have been made toward increasing the frequency band width of the known slot antennas. However, all of them suffer detriment either to stability as exemplified by variable directive characteristics in such cases where the radiating orifices are subject to relative phase differences or else in an economical sense, and at best have afforded only a relatively small increase in band width.

It is an object of the invention to provide an antenna having the advantageous construction of the known slot antenna and an effectively wider frequency band.

It is another object of the invention to provide a slot antenna having the feed point thereof so located that complementary inductive and capacitive reactance components in conjunction with apportioning of the radiation resistance component are connected to the associated transmission network to present a substantially constant impedance characteristic over a wide frequency band.

It is a further object of the invention to provide an antenna having wide frequency band characteristics and especially suitable for installation in restricted space, such as the wingtips of aircraft and the like.

Another object of the invention is to provide an antenna having a substantially constant directivity characteristic over a wide frequency hand.

These and other objects of the invention which will appear as the specification progresses are attained in a receiving or transmitting antenna according to the invention by providing at least one conductive element having two closely parallel unequal length elongated apertures associated therewith, the frequency, and consequently the wavelength, relationship with respect to the length of the apertures being such that the shorter aperture exhibits inductive reactance while the longer aperture exhibits capacitive reactance at the desired operating frequency. The conductors of a two-conductor transmission line are connected individually across the edges of the apertures. In the preferred embodiments, the apertures are in the form of elongated slots and a coaxial transmission line is arranged between the slots with the central conductor thereof connected across the longer slot and the sheath conductor connected across the shorter slot or vice versa, the return path to ground passing from the outer conductor of the coaxial transmission line through the intermediary of a quarter wavelength balun sleeve. The input impedance of the antenna thus obtained is substantially constant over a very wide frequency band.

The invention will be more completely understood by reference to the following description and the accompanying drawing forming part of the specification and in which:

Fig. 1 is a basic arrangement of an antenna according to the invention;

Fig. 2 is a cross-sectional view of the antenna illustrated in Fig. 1;

Fig. 3 is an alternate embodiment of an antenna according to the invention;

Fig. 4 is a cross-sectional view of the antenna illustrated in Fig. 3;

Fig. 5 illustrates the application of the basic antenna of Fig. l to installation in an aircraft wing;

Fig. 6 illustrates how the fundamental radiator illustrated in Fig. 1 or Fig. 3 may be applied to a tower antenna;

Fig. 7 is a cross-sectional view of the antenna illustrated in Fig. 6;

Figs. 8 and 9 are detailed illustrations of the construction of a slot antenna according to Fig. 6;

Figs. 10 and 11 are illustrations of alternate embodiments of the arrangement shown in Figs. 8 and 9;

Figs. 12 and 13 are illustrationsof the application of the basic arrangement shown in Fig. 1 to circular slot antennas;

Fig. 14 is an illustration of the alternate feed system of Fig. 3 applied to the circular slot antenna of Figs. 12 and 13; and

Fig. 15 is a graphical representation of the performance of an antenna according to the arrangement shown in Figs. 1 and 2.

Referring to Figs. 1 and 2, there are shown elevation and cross-sectional views respectively of an antenna structure according to the basic principles of the invention. This particular embodiment of the invention is intended for installation on a conductive surface element 20, which in practice might be constituted by the external surface of an airplane or other vehicle. Theoretically the conductive surface element extends to infinity butpractically there are finite dimensions for each application which will provide the desired results. Such a finite dimensioned element is sometimes termed a semi-infinite element, although the term is not widely used. Two hollow cylindrical conductors 22 and 24 of unequal length are arranged above element 20 by means of conductive members 26 and 28. Conductors 22 and 24 are supported by members 26 and 28 with the longer conductor 24 above conductor 22. The ends of conductors 22, 24 are electrically interconnected by means of members 26, 28. The latter members need not be solid and in fact are preferably hollow since high frequency currents are predominant near the outer surfaces thereof. Members 26, 28 also serve to space conductors 22, 24 from member 20 and each other to provide elongated apertures 31 and 32 of unequal dimensioning. For connecting the desired utilization apparatus, a coaxial transmission line 34 having a central conductor 35 and a sheath 36 is led into the antenna as shown. The central conductor 35 of transmission line 34 extends into conductor 22, which in effect then becomes the sheath conductor of the transmission line. Conductor 35 is led through an aperture 37 in conductor 22 to a tapering connector element 38 electrically connected to conductor 24. These connections of transmission line 34 to conductors 22, 24 are preferably made, at the center as shown, but it may be desirable or even necessary to connect at points. off. center in order to properly match irnpedances. The impedance values of the arrangement shown are maximum at the center of and tend toward zero at the ends of the slots 31 and 32. It should be understood that it is within the scope of the invention to arrange a compensating stub transmission line within conductor 24 and connect the center conductor of the stub transmission line to conductor 35. The length of the stub together with the type of termination, i. e., open or short circuited, wiil determine the coupling to slot 32. Thistypc of coupling is described in greater. detail in copending application Serial No. 612,685 filed August 24, 1945 of Nils E. Lindenblad, since issued on October 30, 1951 as U. S. Patent 2,573,460. Aperture 31 formed by element 20 and conductor 22 is dimensioned with respect to the operating frequency band to provide an inductive reactance component as compared to a capacitive reactance component provided by aperture 32 existing between conductors 22 and 24. Optimum results of this band widening effect are obtained when the inductive and capacitive components have substantially equal portions of the radiation resistance component combined therewith at the midband frequency and when the radiation resistance component is equal to the square root ofthe inductance and capacity quotient.

A streamlined version of the basic arrangement may be constructed as illustrated in Figs. 3 and 4, in which an alternate feed system is also illustrated. A thick, hollow fin member 40 is arranged in conjunction with conductive surface element 20 to provide two slots 41, 42 of unequal length. The frequency, and consequently the wavelength, relationship to the length of these slots is such that across the inner and shorter slot the impedance is inductive while it is capacitive across the outer and longer slot. The height, cross section and slot lengths are designed to provide the best approach possible to the condition R= /m where R is the radiation resistance, L is the inductive reactance and C is the capacitive reactance of the arrangement. The coaxial transmission line 34 is now snaked through, inside the intermediate member 48 between the slots of fin member 49. Center conductor 35 is led through an aperture 43 and connected across long slot 42 to What may then be called the outer and capacitive member. The extremity of outer conductor 36 is connected by a lead 45 which is led through an aperture 47 and connected across slot 41 to member 20. The return path to ground for the current coming through outer conductor 36 is coupled to the intermediate conductor 48 at the edges of apertures 43 and 47 by means of a balun sleeve 49 which surrounds the end of transmissionline 34 for a distance of-substantially a quarter wavelength at the midband operating frequency and is connected to the sheath conductor 36 at the end of the sleeve remote from the end of transmission line 34. Center conductor 35 is then connected to a radiating portion of the antenna which is capacitive, while sheath conductor 36 is connected to a radiating portion which is inductive. Member 40 may be solid except for provisions permitting transmission line 34 to be ted through. However, in the interest of reducing the cost of manufacture and the weight of the finished article, member 40 is preferably made in the form of a hollow casting, and intermediate conductor 48 of member 40 may be arranged to serve as balun sleeve 49 if desired. Member 40 is preferably streamlined especially if the antenna is to be employed on aircraft. However, the streamlining is advantageous not only for aerodynamic reasons but for electrical reasons as well. The current distribution contours of the currents induced in the surface of member 40 will be more uniformly distributed in a streamlined version as shown in Fig. 3 than with the rectangular form shown in Fig. 1.

I Should be oted. that an arrangement such as shown are-mes in Fig. 3 inay be streamlined in the, direction ax al cf the slots or perpendicular to the plane of the slots as dictated by the required installation on the aircraft. In this manner, a highly efficient antenna with low aerodynamic drag can be evolved. The principles of the invention are not confined to an externally mounted protuberant structure, however, but may be most advantageously applied to flush mounting on aircraft and other are installed in the wing surfaces 51 and 53 of aircraft with the slots lying in the surface of the wing. For slots situated near the edge of the wing, within, for example, one-eighth wavelength, the feed arrangement shown in connection with Fig. 1 may be used to advantage. Where,

'ghowever, the slots are located centrally of a large area of conductive sheeting, the balanced feed arrangement, as described in connection with Fig. 3 must be employed. Such an arrangement is by no means limited to aircraft structure either. Referring to Figs. 6 and 7, there is "I: shown an adaptation of the basic structure of the invention to a broadcasting antenna system comprising a tower 60 having four radially arranged hollow conductive wing portions 61-64. Each of these Wing portions has three sets of wide-band slots (id-66 therein arranged one set above another and energized as described in connection with either of the arrangements shown in Figs. 1-4. Such a construction may be made with three or more Wing portions. For example, a tower with four wing portions could be used in television broadcasting with the slot antennas of each section connected in parallel and as outlined in U. S. Patent 2,245,693, issued June 17, 1941,

to Nils E. Lindenblad. Other modes of operation will be suggested to those skilled in the art. In any event, there will be a further broadening of the constant impedance frequency band width due to line compensation which occurs in such combinations.

The slots of the antenna according to the invention are preferably spaced at a distance substantiallyless than one-eighth of the operating wavelength. The closer together the better so long as the conductive material between slots is suflieient to carry the induced currents.

Obviously, ample room must be permitted for practicable types of transmission line connections and so forth.

Where the slots are recessed in the surface of a vehicle or antenna tower structure, they are preferably backed with a cavity resonator in order to provide the inductive and capacitive reactance components which are afforded by the use of the large conductors of the basic embodiments. Referring to Figs. 8 and 9, there is shown an arrangement with a long slot 82 and a short slot 34 arranged in a conductive surface element with a single structure 86 backing both slots 82 and S4 constituting a cavity resonator in conjunction therewith. A coaxial transmission line 88 having a balun sleeve 89 is led into the cavityresonator between the slots and connected across the slots at ,or near to the center thereof as described in connection with the arrangement of Figs. 3 and 4. A more detailed description of the balun shown and other practical forms of baluns may be had by reference to U. S. Patent 2,231,839 to Nils E. Lindenblad. Obviously, the split sheath conductor type of balun described in U. S. Patent 2,454,907 to George H. Brown may also be adapted to the purpose. Alternatively, as shown in Figs. 10 and 11, each of the slots may be backed by individual structures 92 and 94 constituting separate cavity resonators, and transmission line 88 is again connected across slots'81, 82 at or near the Cen er thereof. The latter arrangement is to be preferred since tuning of the individual components is effected by tuning the cavity in known manner, whereas in the former arrangement the slot dimensions must be varied.

It should be understood that the slots of an antenna structure according to the invention may either be open or covered with dielectric material or even completely filled with a suitable dielectric material, the change in characteristics of the antenna being taken into account in the design of the slots and covers.

Referring to Figs. 12 and 13, there is shown an adaptation of the principles of the invention to the ring-shaped slot antenna. Whether a ring-shaped slot antenna is fed in phase quadrature as shown in the drawing or in opposition as in other applications, it can be greatly improved by employing two concentric ring slots 122, 124 backed by conductive surface structures 131 and 133 constituting members forming cavity resonator elements in conjunction with conductive surface element 120. The resultant cavity resonators are of such dimensions that one slot presents inductive reactance and the other capacitive reactance. Either the inner or outer slot may be made inductive or capacitive as desired. However, it will be obvious that a simpler and more advantageous arrangement is had when the inner slot is made inductive. If both slots have about the same radiation resistance and if the inductance and capacity values are so proportioned that the radiation resistance of each slot equals the square root of the inductance-capacity quotient, an approach toward constant impedance over the entire frequency band will be obtained.

In practice, a wide band ring-shaped slot antenna is preferably formed by a drum member having a circular portion 141, a cylindrical wall portion 143, and an annular lip portion 145, leaving a central aperture in which a circular plate member 147 is arranged on a support member 149 fastened to plate portion 141. Plate member 147 is smaller in diameter than the inner diameter of lip memher 145 having an annular slot 122. Slot 122 of cavity resonator 131 is set flush with the conductive surface element 120 by means of another circular base portion 151 to which support 149 is aflixed and which is fastened to element 120 by means of a wall portion 153. Slot 124 is formed between wall portions 143 and 153, between which wall portions and plate portions 141 and 151 a second cavity resonator is formed.

The outer cavity is coupled to a plurality of transmission lines 161 having a sheath conductor 163 connected to wall portion 153 and a central conductor 164 connected to wall portion 143 as shown. Variations in the coupling of course should be obvious to one skilled in the art. For example, the degree of coupling may be varied by moving the points of connection along wall members 143 and 153 parallel to the axis of the drum members 131, 133, or the axis of support 149. Also, the connecting points may be found on members 141 and 151 depending on the im pedance level to be matched and the desired mode of operation.

An alternate feed arrangement, shown in Fig. 14, employs the balanced connection afforded by the incorporation of a balun sleeve 169 having a length of the order of a quarter wavelength at the midband frequency. The end of balun sleeve 169 remote from the end of transmission line 165 is connected to sheath conductor 167, thereby isolating the end of sheath conductor 167 from ground in known manner. Balanced feed connections from the ends of conductor 165 and 166 then connect to the edges of slots 122 and 124. The transmission line is preferably arranged inside wall 143 and below wall 141, in which manner the cavity structures are equally affected by the inclusion of balun sleeve 169. Transmission line 165 is then led out of structure 133 near outer part 149 in order to minimize any deleterious effects due to the presence of the line within the cavity structure.

A graphical representation of the results obtained with an antenna according to the invention is shown in Fig. 15. Standing wave voltage ratio is plotted against the oper- *6 ating frequency for different values of slot length ratio. Curves 171, 172 and 173 were obtained from a rough model constructed along the lines of the arrangement shown in Figs. 1 and 2. The interesting part of the curves is below the 2-1 standing wave ratio line, since that figure is usually accepted as good tolerance. The other arrangements, however, will provide results at least as good as those indicated in the graph.

The radiation pattern obtained with the antenna according to the invention exhibits substantially constant directivity over a wide frequency band since the spacing between slots is substantially less than one-eighth wavelength, thereby avoiding any combining eflfect such as found in arrays of radiator or receptor elements.

The invention claimed is:

1. An antenna system including a conductive structure having two elongated apertures therein lying substantially side by side, one of said apertures being longer than the other, and means to couple conductors of a transmission line to said conductive structure across at least one of said apertures.

2. An antenna system including a conductive structure having two elongated slots therein arranged in substantially parallel relationship, one of said slots being longer than the other, and means to couple conductors of a transmission line individually to said conductive structure across at least one of said slots.

3. An antenna system including a conductive structure having two elongated apertures therein lying substantially side by side, one of said apertures being longer than the other, and means to couple the conductors of a twoconductor coaxial transmission line individually to opposing sides of the longer of said apertures, the sheath conductor being coupled between said apertures and the center conductor being connected at the edge of the longer one of the apertures opposite said connection of the sheath conductor.

4. An antenna system including a hollow metallic surface structure having two elongated apertures therein lying side by side, one of said apertures being longer than the other, means to couple the conductors of a balanced transmission line individually to said structure at the remote edges of said apertures, and conductive wall members arranged within said structure in back of said elongated apertures and constituting cavity resonator means in conjunction therewith.

5. An antenna system including a hollow metallic surface structure having two elongated slots arranged in substantially parallel relationship therein and of unequal length, means to couple conductors of a balanced transmission line individually to the edges of said slots remote from each other, and conductive wall members arranged within said structure in back of each of said slots and congtituting cavity resonator means in conjunction therewit 6. An antenna system including a hollow metallic surface structure having at least two elongated slots therein lying substantially side by side, one of said slots being longer than the other, means to couple conductors of a balanced transmission line individually to the two sides of said slots remote from each other, and conductive wall members arranged within said structure in back of said elongated slots to constitute a cavity resonator in conjunction therewith.

7. An antenna system including a conductive surface element, a pair of elongated cylindrical conductors of unequal length, the shorter of said conductors being hollow for at least part of the length thereof, said conductors 8. An antenna system including a conductive surface element having a pair of concentric annular slots therein, conductive surface members arranged on one side of said conductive surface element behind the inner of said slots,

to form a cavity resonator therewith, further conductive surface members arranged on said one side of said conductive surface element behind the outer of said slots to form a cavity resonator therewith, means to couple a two-conductor transmission line to said antenna system, one of said conductors being coupled to one of the first said conductive surface members and the other of said conductors being coupled to one of said further conductive surface members.

9. An antenna system including a conductive surface element, a pair of elongated cylindrical conductors, one of said conductors being hollow for at least part of the length thereof, said conductors being arranged in parallel relationship to each other and to said conductive surface element, conductive surface members connecting the ends of said conductors to each other and to said conductive surface element, said conductive surface member spacing said conductors above said conductive surface element and having a configuration providing an elongated slot between said conductors and a shorter elongated slot between said one conductor and said conductive surface element, and means to couple a two-conductor transmission line individually to said conductors at substantially the midpoints thereof, said transmission line being led through said hollow portion of said one conductor.

An antenna system including a conductive surface element having a pair of concentric annular slots therein, conductive surface members arranged on one side of said conductive surface elements behind the inner of said slots to form a cavity resonator therewith, further conductive surface members arranged on said one side of said conductive surface element behind the outer of said slots to form a cavity resonator therewith, means to couple a two-conductor transmission line to said antenna system, one of said conductors being coupled to said conductive surface element between said concentric slots and the other of said conductors being coupled to said conductive surface element at the outer edge of said outer slot.

11. An antenna system including a conductive surface element having a pair of elongated slots of unequal length therein, said slots being arranged in parallel relationship to each, other, and means to couple a two-conductor transmission line individually to the edges of the longer of said slots at substantially the midpoint thereof.

12. An antenna system including a conductive surface element having an elongated slot therein, and having therein a further elongated slot arranged parallel to the first said slot, said further slot being shorter than the first said slot, and means to couple a two-conductor transmission line across the first said slot at the edges thereof and substantially at the mid points of said edges.

13. An antenna system for radiating wave energy within a wide frequency band, including a conductive surface element having a pair of elongated slots arranged in substantially parallel relationship therein, the dimensions of said slots having values at which one of said slots exhibits an inductive reactance characteristic at the midband operating frequency and the other exhibits a capacitive reactance characteristic at said frequency, and means to connect utilization apparatus across one slot at opposing edges thereof.

14. An antenna system including a hollow metallic surface structure having at least two elongated slots therein lying side by side, one of said slots being longer than the other, means to couple the conductors of a two-conductor transmission line individually to the edges of said slots remote from each other, and a sleeve member having a length substantially equal to a quarter wavelength at the midband operating frequency surrounding the end of said transmission line and connected at one end to one of said V 8 conductors, and conductive wall members arranged within said structure in back of said elongated slots to constitute a cavity resonator in conjunction therewith.

15. An antenna system including a conductive surface element, a pair of elongated tubular conductors of unequal length, the shorter of said conductors being hollow for at least part of the length thereof, said conductors being arranged in parallel relationship to each other and to said conductive surface element, said shorter conductor being nearer to said conductive surface element, conductive surface members connecting the ends of said conductors to each other and to said conductive surface element, and means to couple the conductors of a two-conductor coaxial transmission line individually to the longer of said tubular conductors at substantially the midpoint thereof and to said conductive surface element, said shorter conductor being connected to the sheath conductor of said coaxial transmission line at a point substantially one-quarter wavelength at the midband operating frequency from the end of said transmission line.

16. An antenna system including a conductive structure having two elongated apertures therein lying substantially side by side, one of said apertures being longer than the other, and means to couple the conductors of a two-conductor coaxial transmission line individually to opposing sides of the longer of said apertures, the sheath conductor being coupled between said apertures and the center conductor being connected at the edge of the longer aperture opposite said connection of the sheath conductor.

17. An antenna system including a hollow metallic surface structure having two elongated apertures therein lying snbstantially side by side, one of said apertures being longer thanthe other, a transmission line having two conductors balanced with respect to a third conductor, said two conductors being connected to said apertures at the edges thereof farthest apart and said third conductor being connected to said structure between said apertures, and conductive wall members arranged Within said structure in back of said elongated apertures and constituting cavity resonator means in conjunction therewith.

18. An antenna system including a conductive surface element having a pair of concentric annular slots therein, conductive surface members arranged on one side of said conductive surface element behind the inner of said slots to form a cavity resonator therewith, and a coaxial transmission line having a sheath and a center conductor, a sleeve member surrounding the end of said coaxial transmission line and having one end thereof connected to said sheath conductor at a point substantially a quarter Wavelength at the midband operating frequency from the end of said sheath conductor, said conductors being individually connected to the innermost and the outermost edges of said annular slots and said sleeve member being connected to the first said conductive surface members.

19. An antenna system including a conductive sheet element having a pair of elongated slots therein, said slots having different dimensions with respect to the midband operating frequency such that the quotient of the reactance value of the slots is substantially equal to the square of the radiation resistance value of the antenna system, and means to maintain radio frequency potential across said slots.

20-. An antenna system for use in conjunction with a substantially plane conductive surface element of effectively infinite dimensions, including an elongated conductive member arranged parallel to and spaced from said conductive surface element, an elongated conductor spaced from and parallel to said conductive member in a plane normal to said conductive surface element and passing through said conductive member, said conductor having a length greater than said conductive member, conducting a transmission line between said conductive member and said conduct-or substantially at the midpoints thereof.

21. An antenna system for use in conjunction with a substantially plane conductive surface element of effectively infinite dimensions, including at least one elongated conductive member arranged parallel to and spaced from said conductive surface element, at least one conductor spaced from and parallel to said conductive member in a plane substantially normal to said conductive surface element and passing through said conductive member, said conductor having a length greater than said conductive member, conducting elements connecting the ends of said conductor and the ends of said conductive member to each other and to said conductive surface element, and means to couple a transmission line between said conductive member and said conductor substantially at the midpoints thereof.

22. An antenna system for use in conjunction with a substantially plane conductive surface element of effectively infinite dimensions, including an elongated hollow conductive member arranged parallel to and spaced from said conductive surface element, and a conductor spaced from and parallel to said conductive member in a plane substantially normal to said conductive surface element and passing through said conductive member, said conductor having a length greater than said conductive mem ber, conducting elements connecting the ends of said conductor and the ends of said conductive member to each other and to said conductive surface element, and a. transmission line arranged within said hollow conductive member and having the conductors thereof connected to said conductive member and said conductor substantially at the midpoints thereof.

References Cited in the tile of this patent UNITED STATES PATENTS 1,933,941 Taylor Nov. 7, 1933 2,129,711 Southworth Sept. 13, 1938 2,206,923 Southworth July 9, 1940 2,242,023 Cork May 13, 1941 2,283,935 King May 26, 1942 2,400,867 Lindenblad May 21, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,435,988 Varian Feb. 17, 1948 2,488,419 Lindenblad Nov. 15, 1949 2,508,085 Alford May 16, 1950 2,511,931 Masters June 20, 1950 2,513,007 Darling June 27, 1950 2,557,951 De Rosa June 26, 1951 2,661,422 Johnson Dec. 1, 1953 FOREIGN PATENTS 446,441 Great Britain Apr. 30, 1946 611,052 Great Britain Oct. 25, 1948 628,584 Great Britain Aug. 31, 1949 647,425 Great Britain Dec. 13, 1950 OTHER REFERENCES Slot Aerials, by D. A. Bell, Wireless World, February 1948, pages 57 and 58. 

